Multipacting, also called multipactoring, is a phenomenon of resonant electron multiplication in a vacuum to which an RF or microwave (MW) field is applied. Multipacting occurs when electrons in the RF or MW field oscillate synchronously and lead to a secondary emission of electrons when hitting electrodes or other surfaces of the enclosure. If the secondary electron yield (SEY), i. e. the average number of electrons emitted by a surface when hit by an electron is larger than one, the number of electrons constantly increases and builds up an electron avalanche, which in turn leads to remarkable power losses and heating of the enclosure walls. Accordingly, due to multipacting it becomes difficult to increase the cavity fields by raising the incident power. In superconductive structures, a large rise of temperature due to multipacting can lead to a thermal breakdown. Also, a heavy bombardment of multipacting electrons may even break ceramic windows in the power feed lines. Due to these problems, there is a strong desire to suppress multipacting in vacuum RF or MW devices.
In principle, multipacting can occur in any device or apparatus where considerable microwave or RF power is used in a vacuum enclosure or cavity. Specifically, multipacting is known to be a serious problem in microwave devices of satellites, such as microwave filters and wave guides. With regard to satellites or other space applications, the power losses and power limitation due to multipacting are extremely disadvantageous as for obvious reasons, power supply is severely limited in space. However multipacting also adversely affects the operation of particle accelerators, such as linear particle accelerators used in medical radiotherapy devices, or accelerators used in physics or material sciences.
In prior art, different approaches are known to suppress the undesirable multipacting effect. Since multipacting is essentially an electron resonance effect, in one approach one seeks to design the field enclosure or cavity such as to avoid resonance of the high frequency electric field to be used. For example, if the electron runtime between two opposite electrodes happens to be an odd multiple of half a period of the field, the electrons will acquire a net acceleration between the electrodes and can thus build up an electron avalanche. Accordingly, one approach to suppress multipacting is to avoid combinations of driving field and cavity geometries that would lead to such resonances. However, this greatly limits the variety of possible device geometries and applicable electromagnetic fields and is thus an undesirable limitation for the design of a device.
A second approach for suppressing multipacting is a rather microscopic one: as mentioned before, the multipacting effect occurs when the secondary electron yield (SEY) is larger than one. Accordingly, if one is able to decrease the SEY, multipacting can be effectively suppressed or massively reduced. There have been attempts to decrease the SEY by using special surface coatings, such as titanium nitrate and others. However, such coating techniques have the problem that they tend to increase the RF or microwave losses. Also, in some cases the coatings are not stable in time, particularly when they are temporarily exposed to air.
A further microscopic approach for lowering the SEY is based on an artificial microscopic roughening of the inner surface of the enclosure. The surface roughness acts as a kind of local electron trap as it reduces the probability that secondary electrons released from the surface can actually escape. Due to the rough surface structure, released secondary electrons may be immediately caught again by a protruding portion of the surface such that it does not contribute to the buildup of an electron avalanche. While artificial surface roughening has in fact proven to allow a reduction of SEY, unfortunately, it also considerably increases microwave and RF losses, which in particular with regard to applications in satellites or space technology in general is disadvantageous.
Accordingly, there still exists a need for an apparatus having means for at least partially suppressing multipacting, a method for making such apparatus and a method for suppressing multipacting.